Hanging out with Steve Platek will make you yawn. He’ll get you thinking about yawning, reading about yawning, and sooner or later, your mouth’s gaping. You can’t help it. “My favorite way to induce a yawn,” Platek says, “is a video clip of a good yawner paired with yawn audio.” Platek, a cognitive neuroscientist at Drexel University, alternately describes yawning as “a primitive unconscious mechanism” or something that’s “sweet,” “totally cool” or “awesome.” And he’s finally figuring out why it’s contagious.

Scientists (and everybody else) have known for decades that yawns are contagious, but they’ve never known why. Some think it’s an unconscious mirror effect – someone yawns, you yawn in response almost like a reflex. But Platek says he thinks it has to do with empathy. The way he sees it, the more empathetic you are, the more likely it is that you’ll identify with a yawner and experience a yawn yourself. In a recent study, Platek looked at contagious yawning in people with “high empathy,” “low empathy” and everything in between. He found that higher empathy meant more yawn-susceptible and lower empathy meant more yawn-immune.

But that wasn’t proof enough. So Platek put volunteers in M.R.I. machines and made them yawn again and again to pinpoint the areas of the brain involved. When their brains lighted up in the exact regions of the brain involved in empathy, Platek remembers thinking, “Wow, this is so cool!”

Some yawning researchers – of which there are few – have identified many types of yawns. There’s the contagious yawn, the I’m-tired yawn and the I-just-woke-up yawn. There’s the threat yawn, which is the my-teeth-are-bigger-than-yours yawn that’s so popular with primates. (“People do it, too,” says Platek, “but unfortunately, we don’t have scary teeth anymore.”) There’s also the sexual yawn. (One scientist claims that yawns are used in seduction.)

At some point, you have to wonder: why study yawning? It’s quirky, interesting, but not important, right? Wrong, says Platek. Nearly every species on the planet yawns: insects, fish, birds, reptiles, mammals. “Yawning is such a primitive neurological function,” Platek says, “it’s a window into what happened during the evolution of the brain.”

The good thing about yawning is that it’s not boring. “Scientists like me usually go to conferences and give talks about technical mumbo jumbo,” Platek says. “The audience always yawns, and we’re up there thinking, Oh, man, they’re so bored! But when I give a talk about yawning and they yawn, I think: Sweet! They’re paying attention!”

Several years ago, I took a remarkable psychology class from a professor whose interest in the field seemed to be motivated, at least in part, by a desire to unearth new tricks for beguiling the opposite sex. Ethical implications aside, his lectures were generously garnished with anecdotes that held his wide-eyed freshmen audience in rapt attention. Among the tips he shared was a psychologically based method for casually gauging just who might be checking you out in a crowded place: yawning. The theory goes that if you yawn, anyone who’s surreptitiously watching you won’t be able to resist yawning too, giving themselves away (and marking them as a susceptible target for wooing). (Pics)

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The trick is based on a real phenomenon called “contagious yawning,” in which seeing, hearing, or even thinking about someone else yawning causes you to follow suit. The biological function of yawning is debatable and there are several kinds of yawns that may signal everything from fatigue to aggression. But contagiously opening your yapper in a yawn seems to be distinct from doing so spontaneously, based on the fact that the former does not occur in all animals. In other words, plenty of animals yawn, but not all animals mimic one another’s yawns.

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Whether or not seeing someone else’s jaws gaping wide gives you the uncontrollable urge to join in may say more about your personality than you realize. Roughly half of people yawn when watching videos of others yawning.

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Some researchers hypothesize that contagious yawning is more common in people with greater empathy — the ability to recognize and share other people’s emotions — an idea that has gotten quite a bit of press lately.

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Developmental clues support the empathy theory for contagious yawns. Children younger than about four years old, it seems, yawn spontaneously but don’t catch contagious yawns from adults or from one another. Neither do many older children with autism spectrum disorder, who have an impaired ability to communicate, socialize, and empathize, according to a paper published this summer in Biology Letters.

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The authors found that children with autism yawned spontaneously about as often as children without autism. But when watching videos of yawning adults, children with autism yawned fewer times than other children. They believe this supports the claim that contagious yawning is based on the capacity for empathy.

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Indeed, studies of what’s actually happening in the brain when we see others yawn also seem to support the empathy explanation. For example, in 2005, researchers used an imaging technique called fMRI to see what happens in the brain when people see others yawn. The images showed that in the person seeing the yawn, there was increased activity in the part of the brain involved in self-processing, a region believed to be related to empathy.

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Contagious yawning is not a strictly human habit, though. Research shows that a couple animals related to our species, like chimps, do it too. In one study, British and Japanese researchers showed six female chimps videos of other female chimps that were either yawning or just opening their mouths.

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A third of the chimps yawned significantly more when watching other chimps yawn, while none of them yawned when watching the non-yawns.

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So, yes, it seems that my professor was on to something with his yawning scheme. Not only does research show that yawning is contagious, it suggests yawning weeds out the starers who are a bit less empathetic (too bad the yawning trick won’t weed out the chimps, too).